1. Technical Field
The present disclosure relates generally to radio frequency (RF) integrated circuits, and more particularly, to complementary metal oxide semiconductor (CMOS) RF power amplifiers with high linearity across a wide range of burst signals in WiFi applications.
2. Related Art
Wireless communications systems are utilized in a variety contexts involving information transfer over long and short distances alike, and a wide range of modalities for addressing the particular needs of each being known in the art. As a general matter, wireless communications involve an RF carrier signal that is variously modulated to represent information/data, and the encoding, modulation, transmission, reception, de-modulation, and decoding of the signal conform to a set of standards for coordination of the same.
In the local area data networking context, WLAN or Wireless LAN, also commonly referred to as WiFi as well as 802.11 (referring to the governing IEEE standard), is the most widely deployed. The later, more advanced WiFi standards such as 802.11ac, and the previous 802.11n and 802.11a standards on which it was based specify an orthogonal frequency division multiplexing system where equally spaced subcarriers at different frequencies are used to transmit data. Several computer systems or network nodes within a local area can connect to an access point, which in turn may provide a link to other networks and the greater global Internet network. Computing devices of all form factors, from mobile phones, tablets, and personal computers now have WiFi connectivity, and WiFi networks may be found everywhere.
As is fundamental to any wireless communications systems, a WiFi network interface device includes a transceiver, that is, a combined transmitter and receiver circuitry. The transceiver, with its digital baseband system, encodes the digital data to an analog baseband signal, and modulates the baseband signal with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the digital data represented by the baseband signal. An antenna connected to the transceiver converts the electrical signal to electromagnetic waves, and vice versa. In most cases, the transceiver circuitry itself does not generate sufficient power or have sufficient sensitivity necessary for communications. Thus, additional circuits are referred to as a front end is utilized between the transceiver and the antenna. The front end includes a power amplifier for boosting transmission power, and/or a low noise amplifier to increase reception sensitivity.
RF power amplifiers utilized in WiFi systems ideally have linear performance, which is described in terms of the error vector magnitude (EVM) of the transmitted signal. In order to conserve energy, the power amplifier is turned on and off in accordance with the transmit signal burst applied to its input. However, such switching generates transient current, voltage, power gain, phase, and so on. In particular, the edges of ramping signals results in a deterioration of EVM, also referred to as dynamic EVM, which is understood to differ from static EVM, where the control signal applied to the power amplifier is in a continuously on state. In addition to the transient signals attributable to the dynamically switching currents and voltages, thermal properties of the transistors in the power amplifier circuitry also contribute to transient signals.
In the publication “Static and Dynamic Error Vector Magnitude Behavior of 2.4-GHz Power Amplifier”, Sang-Woong Yoon, IEEE Transactions on Microwave Theory and Techniques, Vol. 55, No. 4, April 2007, a thermal effects influence on dynamic EVM was presented, and compared to static EVM. The explanation was limited, however, to the output power level change during burst, which is understood to be equivalent to gain change. Conventional communications systems can readily demodulate signals with the small power level variations described. While the root cause of thermal heating affecting dynamic EVM was explored, only a partial explanation was proposed. Subsequently, in the publication “Self-heating and Memory Effects in RF Power Amplifiers Explained Through Electro-Thermal Modeling”, Wei Wei, et al., in NORCHIP 2013, November 2013, it was discovered that there are both amplitude-amplitude (AM-AM) and amplitude-phase (AM-PM) distortions that may be found in the modulated signal as a result of thermal effects. These distortions were found to influence the level and phase of the inter-modulation products. According to the described simulation, even below a 50 kHz two-tone spacing, there was no difference in left- and right-side products of intermodulation distortion. In actual implementation, envelope variations may be at a rate of several megahertz or tens of megahertz. Such fast variation of frequency is not understood to cause a fast and large variation of power amplifier transistor temperatures within the semiconductor die. In WiFi systems utilizing conventional gallium arsenide (GaAs) or silicon technologies, the thermal time constant for power amplifier transistor stages can range from several microseconds to several tens of microseconds.
A technique for compensating for power amplifier transients at the beginning of the transmission burst is disclosed in U.S. Pat. No. 8,260,224 to Doherty et al. This technique is understood to require a pulsed “pre-heating” by a high current for over hundreds of microseconds before the RF signal burst with consecutive current shaping. Such “pre-heating” is understood to be impractical for WiFi signals, as incoming RF signal bursts are dependent on multiple factors according to the network protocol. Problematically, the delay of the RF transmit signal is understood to result in a substantial reduction in data throughput. Furthermore, an additional control input/timing is needed, and though this is typically lacking in existing WiFi platform solutions.
U.S. Pat. App. Pub. No. 2013/0307625 to Hershberger et al. disclosed a bias boost circuit that is applied to the base of the RF transistor in the WiFi power amplifier. A constant bias is applied during the RF signal burst, in addition to an exponentially decaying boost current that is applied at the beginning of the burst to compensate for RF transients. Although this technique may be suitable for power amplifiers implemented with bipolar transistors, in CMOS-based power amplifiers, a high level of transients may be generated, and degrade dynamic EVM further.
U.S. Pat. App. Pub. No. 2013/0127540 to Kim et al. disclosed a power amplifier with phase compensation circuitry. Specifically, phase compensation over the RF signal power level with a pre-distortion for linear power amplifiers is disclosed, but is not understood to be useful for minimizing dynamic EVM variations at signal burst edges.
A transient compensation circuit particular to WiFi power amplifiers is disclosed in U.S. Pat. No. 7,532,066 to Struble, et al. A current steering circuit is used to add current at the beginning of an RF signal burst along the same lines as Hershberger. However, dynamic degradation is not considered, and the solution appears limited to power level dependence.
In a publication entitled “Front-end Modules with Versatile Dynamic EVM Correction for 802.11 Applications in the 2 GHz Band”, Samelis, et al., 2014 IEEE Topical Conference on Power Amplifiers for Wireless and Radio Applications (PAWR), January 2014, test results of a dynamic EVM compensation circuit implemented with silicon-germanium (SiGe) heterojunction bipolar transistor (HBT) power amplifiers for a WiFi front end circuit are disclosed. Thermal dependence was indicated as the root cause of dynamic EVM for different burst conditions, but only fairly short burst windows of approximately 176 microseconds were considered, which are typical for mobile applications. In the proposed circuit, digital settings would be needed during preliminary calibration at different power levels. Furthermore, the proposed circuit is understood to be unsuitable for increased bias voltages that are typical of more recent modulation schemes such as those specified in the 802.11ac standard.
Along these lines, recent WiFi systems implementing 802.11n and/or 802.11ac may employ a wider burst of up to several milliseconds to transmit a larger amount of data as would be typical in access point or router operation. The more substantial thermal issues along with higher transmit power levels complicate dynamic EVM compensation circuits. While suitable for such high power applications, GaAs semiconductor material has approximately three times the thermal resistance of silicon material, so circuits fabricated therewith are understood to be more prone to transients and dynamic EVM deterioration at different burst conditions.
Accordingly, there is a need in the art to address the problem of dynamic EVM over the entire burst duration. That is, there is a need in the art for RF power amplifiers with high linearity across a wide range of burst signals in WiFi applications.